1. Field of the Invention
The present invention relates to a liquid material supply system, which may be used in a car assembly plant to coat automotive components or works with a constant amount of a sealing compound or other liquid material, or to fill them with a constant amount of an adhesive, grease or other liquid material.
2. Description of Related Art
Generally, in a car assembly plant, a plunger pump, which is a high pressure pump, sucks a sealing compound, an adhesive or other liquid material from a storage tank and supplies it through supply lines to dispensers, each of which is connected to one of the lines. The dispensers coat or fill works with the liquid material. In such a system, a plunger pump or another high pressure pump is used to supply liquid material to one or more distant places.
FIG. 3 of the accompanying drawings shows a conventional system for supplying a sealing compound to distant dispensers 103, one of which is shown, for coating works with the compound. This system includes a storage tank 108, which is connected to a plunger pump 101. The pump 101 is connected through supply lines 102, one of which is shown, to dispensers 103, one of which is shown, each connected to one of the lines 102.
Each supply line 102 is fitted with a flow regulating valve 104 as a pressure reducing valve. The supply line 102 consists of a primary supply line 102xe2x80x2, which is high in pressure, on the upstream side of the pressure reducing valve 104 and a secondary supply line 102xe2x80x3, which is low in pressure, on the downstream side of this valve 104. The pressure in the primary supply line 102xe2x80x2 is kept at a high value of about 15 MPa (150 kg/cm2). The secondary supply line 102xe2x80x3 is fitted with an air-operated valve 105 as an on-off valve.
The plunger pump 101 sucks the sealing compound from the storage tank 108 and supplies it under high pressure to the supply lines 102, from which it is supplied to the respective dispensers 103. The dispensers 103 discharge the sealing compound directly onto works so as to coat or fill them with a constant amount of sealing compound.
The flow regulating valve 104 reduces the pressure in the secondary supply line 102xe2x80x3, which is the proper supply pressure for the associated dispenser 103, to a value lower than that in the primary supply line 102xe2x80x2 for the following reason.
Because the dispenser 103 is mounted on a robot (not shown) or the like, it is preferable that the dispenser 103 be small in size, light in weight and able to discharge a constant amount of liquid material. The dispenser 103 may be a small-capacity single-shaft eccentric screw pump. It is necessary that the discharge pressure of the dispenser 103 be very lower than that of the high pressure pump 108. In other words, there is an upper limit to the supply pressure for the dispenser 103.
The dispenser 103 is fitted with a pressure sensor 106 near its inlet port 103a. The sensor 106 detects the pressure substantially at the port 103a and outputs a pressure signal to an electromagnetic valve 107, which is an on-off valve. This valve 107 controls the switching operation of the air-operated valve 105 depending on the pressure substantially at the dispenser port 103a. The air-operated valve 105 is closed if this pressure, which is the value detected by the sensor 106, is higher than a set upper limit, which may be 0.7 MPa. This valve 105 is opened if the pressure is lower than a set lower limit, which may be 0.3 MPa.
The dispenser 103 intermittently discharges liquid material. In order for the dispenser 103 to discharge a sufficient amount of liquid material every time it starts to discharge liquid material, it is necessary that the pressure in the secondary supply line 102xe2x80x3 be kept high to some extent.
Therefore, as soon as the dispenser 103 stops discharging liquid material, the pressure in the secondary supply line 102xe2x80x3 rises. When this pressure exceeds the set upper limit, the air-operated valve 105 is closed. Thereafter, as soon as the dispenser 103 starts discharging liquid material, the pressure in the secondary supply line 102xe2x80x3 falls. When this pressure falls below the set lower limit, the air-operated valve 105 is opened. Thus, every time the dispenser 103 starts and stops discharging liquid material, the pressure in the secondary supply line 102xe2x80x3 falls below the lower limit and rises above the upper limit. As a result, the air-operated valve 105 frequently closes and opens. This may shorten the life of the air-operated valve 105.
The applicant""s Japanese patent laid-open publication No. 2002-316081 discloses a liquid material supply system including a supply device and a dispenser, which is connected to the supply device by a supply line. The supply line is fitted with a pressure reducing valve, an on-off valve and a buffer pump, which is a single-shaft eccentric screw pump. The pressure reducing valve is interposed between the supply device and the on-off valve. The screw pump is interposed between the on-off valve and the dispenser. The operation of the buffer pump and the on-off valve is controlled, based on the pressure in the supply line between this pump and the dispenser. The use of the buffer pump makes the pressure reducing valve achieve a larger pressure reduction than in the system shown in FIG. 3. This reduces the pressure acting on the dispenser, and prevents liquid from dripping when the dispenser stops and reverses.
As is the case with the system shown in FIG. 3, the on-off valve of the system disclosed in the Japanese publication frequently closes and opens. This may shorten the life of the on-off valve.
In view of the foregoing, the object of the present invention is to provide a liquid material supply system having an on-off valve the life of which is lengthened simply at low cost.
A liquid material supply system according to the present invention includes a supply device, a pressure reducing valve and a discharger. The supply device sucks liquid material from a storage tank or another reservoir, and supplies the sucked material under high pressure. The pressure reducing valve has a pressure reduction ratio that can be set. The discharger discharges a constant amount of liquid material to a work. The outlet port of the supply device is connected to the pressure reducing valve by a primary supply line. The pressure reducing valve is connected to the inlet of the discharger by a secondary supply line. The secondary supply line is fitted with an on-off valve, to which a controller is connected. The supply system also includes a pressure sensor for detecting the pressure substantially at the inlet port of the discharger and outputting a pressure signal to the controller. If the detected pressure exceeds a set upper limit, the controller closes the on-off valve. If the detected pressure falls below a set lower limit, the controller opens the on-off valve. The secondary supply line is also fitted with an accumulator between the on-off valve and the inlet port of the discharger. The accumulator prevents the pressure substantially at the inlet port of the discharger from exceeding the upper limit and falling below the lower limit in a short time with the pressure reduction ratio so set that the pressure is lower than for the full flow through the secondary supply line while the discharger is operating.
The life of the on-off valve becomes shorter as the opening and closing frequency of this valve goes up. The frequency is decreased greatly by the combination of the pressure reducing valve, the pressure reduction ratio of which can be set, and the accumulator.
The internal volume of the accumulator decreases if the pressure in the secondary supply line falls while the discharger is discharging liquid material, with the pressure reduction ratio so set that the pressure is lower than for the fill flow through this line while the discharger is operating. This prevents the pressure in the secondary supply line from falling below the set lower limit. Thus, the accumulator compensates for the shortage of the liquid material supplied to the discharger.
When the discharger stops discharging liquid material, the internal volume of the accumulator increases so as to absorb the rise of the pressure in the secondary supply line, preventing this pressure from exceeding the set upper limit.
Thus, the combination of the pressure reducing valve of which the pressure reduction ratio is suitably set and the accumulator almost prevents the pressure in the secondary supply line from exceeding the set upper limit and falling below the set lower limit. Accordingly, the opening and closing frequency of the on-off valve decreases greatly in comparison with that in the conventional system. This lengthens the life of the on-off valve.
More specifically, if it is possible to suitably set the pressure reduction ratio of the pressure reducing valve so as to adjust the average flow within a certain fixed time, depending on the cycles of discharge and stopping of the discharger, the on-off valve will theoretically be kept open. Therefore, if the flow through the secondary supply line is slightly more than the average flow for safety, the opening and closing frequency of the on-off valve greatly decreases, and the material supply is prevented from being short.
The accumulator varies the supply pressure for the discharger, but does not affect the discharge operation of the discharger because the discharger can supply a work with a constant amount of liquid material.